1. Technical Field
New techniques are disclosed that employ nuclear magnetic resonance (NMR) logging tools to measure time-lapse diffusivity in an observation well of a reservoir undergoing enhance oil recovery (EOR). More specifically, techniques are disclosed which eliminate problems associated with changes in fluid or poor cement bond in the annulus between the well casing the formation which render NMR logs ineffective. The techniques include drilling a deviated observation borehole and using non-magnetic, non-conductive casing without centralizers in the zone of interest thereby ensuring that the casing rests against the formation on the low side of the deviated borehole. The zone of interest is logged with an NMR tool with the pad directed radially at the portion of the casing that rests against the low side of the borehole ensuring that the shallowest zone of interest lies in the formation and not the annulus. Other cased-hole NMR imaging/logging techniques include logging behind completion hardware (casing, sand control screen, etc.) that is non-conductive and non-magnetic for evaluating cement and sand control operations.
2. Description of the Related Art
The development of an oil field or reservoir may include three stages known as primary, secondary and tertiary recoveries. Primary recovery produces oil and gas using the natural pressure of the reservoir as the driving force to push the oil to the surface. Wells are often “stimulated” through the injection of fluids, which fracture the hydrocarbon-bearing formation to improve the flow of oil and gas from the reservoir to the wellhead. Other techniques, such as pumping and gas lift help production when the reservoir pressure dissipates.
Secondary recovery uses other mechanisms, such as gas reinjection and water flooding, to produce residual oil and gas remaining after the primary recovery phase. Tertiary recovery involves injecting of other gases (such as carbon dioxide), heat (steam or hot water), or chemicals to stimulate oil and gas flow to produce remaining residual fluids that were not extracted during primary or secondary recovery phases.
Enhanced oil recovery (EOR) is a type of tertiary recovery. Gas injection is the most common form of EOR and involves the injection of a carbon dioxide (CO2), natural gas, or nitrogen, into the reservoir through an injection well whereupon the gas expands and pushes additional oil to a production wellbore. The injected gas is preferably miscible in the oil to lower the viscosity and improve the flow rate of the oil. Another type of EOR is thermal recovery, which uses heat to improve oil flow rates. Chemical injection is yet another type of EOR where polymers are injected to increase the effectiveness of water floods, or the use of detergent-like surfactants (e.g., alkaline surfactant polymers or ASP) are injected to help lower the surface tension that often prevents oil droplets from moving through a reservoir.
During EOR, it is common to monitor the water saturation changes from an observation well positioned some distance from the injector wells. Water saturation levels are relatively easy to monitor with conventional logging tools, such as induction, pulsed neutron or thermal neutron tools. However, some EOR processes do not involve any changes in water saturation, unlike primary and secondary recoveries where recovered oil is naturally replaced in the reservoir by water. Specifically, when the reservoir is injected with gas in a “miscible gas flood” operation, recovered oil is not predominately replaced by formation water, and therefore current reservoir monitoring techniques that measure the changes in water saturation to calculate the displacement of oil will be ineffective. For example, thermal neutron-emitting tools that measure changes in capture cross section, induction tools that measure conductivity or pulsed neutron tools that measure carbon/oxygen ratio are ineffective when the water saturation level does not appreciably change.
Thus, EOR processes that do not involve changes in water saturation include miscible gas floods and gas-oil gravity drainage projects. Monitoring these gas-based EOR processes requires a direct measure of in-situ oil de-saturation from within an observation well. There are several physical properties of oil and gases used in EOR that can be used to distinguish residual oil from the injected gas, including density differences and differences in hydrogen index (HI—the number of hydrogen atoms per unit volume divided by the number of hydrogen atoms per unit volume of pure water). Gamma-gamma density logs and neutron porosity logs can be used to measure the changes in density and HI respectively, giving a time-lapse measure of oil de-saturation. However, reliance upon changes in HI loses effectiveness if there is little difference in density and HI between the oil and gas. This can occur in miscible gas projects in which the gas properties and gas injection pressures are selected to enhance miscibility. Consequently, another physical parameter distinguishing oil and gas must be measured to calculate the oil de-saturation.
One possible parameter is diffusivity or diffusion coefficient, which refers to the motion of atoms in a gaseous or liquid state due to their thermal energy. Because diffusivity D (m2/s) is dependent on the pore sizes of the formation and well as the fluid in the formation, diffusivity can be used as a gas and oil saturation indicator in addition to a permeability indicator. NMR imaging can provide a good measure of diffusivity. In a uniform magnetic field, diffusivity has little effect on the decay rate of the measured NMR echoes. In a gradient magnetic field, diffusion causes atoms to move from their original positions to new ones, which causes these atoms to acquire different phase shifts compared to atoms that did not move, and will thus contribute to a faster rate of relaxation. Therefore, in a gradient magnetic field, diffusivity is an NMR logging parameter which can provide independent information about the fluids in the formation (e.g., gas and oil saturation) and the structure of the formation. Thus, time-lapse diffusivity measurements of cased observation wells would provide important information about changes in formation fluids over the course of an EOR operation.
Currently, NMR imaging is used routinely in open holes because of its ability to record a real time permeability and porosity log that characterizes the near wellbore region (up to 4″ of depth of investigation). NMR logs are used to predict production rates and assist in the planning of completion and stimulation operations. As noted above, NMR tools can be used to measure diffusivity as well as HI.
However, NMR tools are not being used during completion, production and monitoring of wells because typical metallic completion hardware attenuates the pulsed radio frequency signals used during NMR imaging. Further, in a cased hole, an NMR tool would be ineffective in measuring diffusivity if the cement bond between the casing and formation is poor and/or fluid is disposed in the annulus between the casing and formation. Specifically, a poor cement bond between the casing and formation results in areas of the annulus where voids in the cement are present. Borehole fluid fills these voids, which will change over time from, for example, oil to water. In this case, fluid changes in annulus will completely dominate the time-lapse response of any shallow reading log measurement, such as an NMR log. Measurements made by current NMR tools may not extend beyond the annulus is the annulus exceeds three inches or more in the zone of interest.
Accordingly, while a time-based measure of diffusivity could be used to monitor a miscible gas EOR project, problems associated with the use of NMR tools in cased holes must be overcome. Further, if such problems are overcome, NMR tools could also be used in other cased hole operations.